Method for stimulating well production

ABSTRACT

The production of hydrocarbons from a subterranean hydrocarbonbearing formation containing acid-soluble components, such as one composed at least in part of dolomite or limestone, is stimulated by injecting into the formation a composition comprising an aqueous solution of a mineral acid having dissolved therein a vinylpyrrolidone polymer. The increase in the permeability and porosity of the formation achieved utilizing the method of invention results in a substantial improvement in hydrocarbon recovery.

United States Patent 1191 Tate I i METHOD FOR STIMULATING WELLPRODUCTION Jack F. Tate, Houston, Tex. [73] Assignee: Texaco Inc., NewYork, NY.

[22] Filed: June 25, 1971 [21] Appl. No.: 157,051

Related US. Application Data [63] Continuationin-part of Ser. No.82,182, Oct. 19,

1970, abandoned.

[75] Inventor:

3,434,971 3 1969 Atkins 166/307 x 3,254,719 6 1966 3,294,729 12/19663,294,765 12/1966 Hort et a1 166/307 ux Primary Examiner-Stephen .1.Novosad Attorney, Agent, or FirmThomas H. Whaley; Carl G. Ries [57]ABSTRACT The production of hydrocarbons from a subterraneanhydrocarbon-bearing formation containing acidsoluble components, such asone composed at least in part of dolomite or limestone, is stimulatedby' injecting into the formation a composition comprising an aqueoussolution of a mineral acid having dissolved therein a vinylpyrrolidonepolymer. The increase in the permeability and porosity of the formationachieved utilizing the method of invention results in a substantialimprovement in hydrocarbon recovery.

6 Claims, No Drawings METHOD FOR STIMULATING WELL PRODUCTION CROSSREFERENCE TO COPENDING APPLICATION This-application is acontinuation-in-part of commonly assigned application Ser. No. 82,182filed Oct. 19, 1970 now abandoned.

FIELD OF THE INVENTION .solution of a vinylpyrrolidone polymer and amineral acid.

DESCRIPTION OF THE PRIOR ART The technique of increasing thepermeability of a subterranean hydrocarbon-bearing formation for thepurpose of stimulating the production of fluids therefrom has long beenpracticed in the art. One such method commonly employed is known asacidizing which is widely utilized in treating subsurface calcareousgeological formations, e.g., limestone, dolomite, etc. In the usualwell-acidizing procedure, a nonoxidizing mineral acid is introduced intothe well and under sufficient pressure is forced into the adjacentsubterranean formation where it reacts with acidreactive components,particularly the carbonates such as calcium carbonate, magnesiumcarbonate, etc., to form the respective salt of the acid, carbon dioxideand water. The usual acid employed in such acidization procedures ishydrochloric acid.

During the process passageways for fluid flow are created or existingpassageways therein are enlarged thus stimulating the production of oil,water, brines and various gases. If desired, the acidization may becarried out at an injection pressure sufiiciently great to createfractures in the strata or formation which has the desired advantage ofopening up passageways into the formation along which the acid cantravel to more remote areas from the well bore. The salt formed isextensively water soluble and is readily removed by reverse flow fromthe formation via the well bore.

There are, however, disadvantages attending the use of hydrochloric acidor other similar non-oxidizing mineral acids. For example, these strongaqueous acids have almost instantaneous rates of reaction with thecarbonates contained in the formation. The acid, therefore, necessarilyspends itself in the formation immediately adjacent the well bore solittle beneficial effect is realized at any great distance from the wellbore within the formation under treatment. These strong aqueous acidsalso may cause channeling, cavitation and eventual collapse of theformation immediately adjacent the well bore due to excessively rapidaction of the acid. In addition, subsurface equipment may be damagedseverely by strong acid attack.

One suggested solution to overcome these disadvantages has been the useof retarded acids which consist of a mineral acid and an additive whichemulsifies the acid the combination ofwhich affects the acidizationrate. However, although such emulsified mixtures can be-displaced into aformation before substantial reaction occurs, they have the inherentdisadvantage that when the emulsion breaks and they do react, theyusually react swiftly, often unpredictably, and without substantiallyresolving the problem of cavitation.

A later development in acidizing wells has been the use of potentiallyacidic materials which are capable of producing a strong aqueous acid bymeans of a chemical reaction at a more or less constant rate over aperiod of time and which subsequently reacts wit-h the formation toenhance porosity and permeability. However, with aqueous systemscontaining these potentially acidic material, the effectiveness of theacidizing operation too often is negated in subterraneanhydrocarbon-bearing formations which contain water-sensitive clays orshales. These water-sensitive shales react or interact with the aqueousmedium toswell, which results in decreasing formation permeability,thereby largely offsetting any improvement in permeability which mighthave been realized by the action of the potentially acidic material. Infact, it has been found that the net effect of the swelling ofwater-sensitive clays often may result in an overall decrease information permeability following the acidization.

The use of cross-linked copolymers of polyvinylpyrrolidone andpolyacrylamides, polyurethanes, etc. to give materials which areinsoluble in aqueous mineral acid solutions is described in US. Pat. No.3,3 80,529 to Hendrickson. Such insoluble,.cross-linked polymers areutilized as agents for partially plugging channels developed duringacidization treatment in order to effect acidic attack at greaterdistances from the well bore. In [1.8. Pat. No. 3,434,971 a similaracidization process is described in which a copolymer prepared bypolymerizing acrylamide and N- vinylpyrrolidone in the presence of across-linking agent, such as N,N-methylenebisacrylamide, is disclosed.These copolymers are insoluble in the aqueous mineral acid solutionsemployed in acidization operations and are utilized as dispersions inthe acid solutions. Both of the previously mentioned acidizationprocesses which utilize insoluble, cross-linked copolymers aredistinctly different from the novel method of this invention in which avinylpyrrolidone polymer soluble in aqueous mineral acid solutions isused.

It is therefore, the principal object of the present invention toovercome the defects of the prior art in acidizing fluid bearingformations such as hydrocarbonbearing formations, etc., by providing amethod utilizing the novel acidizing composition of this invention inwhich the distance to which the acidizing composition penetrates theformation before becoming spent is extended, thus providing in-depthacidization.

Another object of this invention is to provide a method of acidizationemploying the novel composition of this invention which is effective inpreventing the swelling of water-sensitive clays and shales.

Another object of this invention is to provide a high viscosityacidizing fluid containing in solution a vinylpyrrolidone polymer whichis stable over long periods of time in which the polymer constituentdoes not hydrolyze or otherwise decompose on storage thus avoiding thesimultaneous reduction in viscosity'and effectiveness inacidization-in-depth operations.

SUMMARY OF THE liNVENTiON This invention encompasses and includes amethod of increasing the production of fluids from a subterraneanfluid-bearing formation having present acidsoluble components and withor without watersensitive clays or shales, comprising injecting down thewell bore penetrating said formation and injecting therefrom into saidformation under a pressure greater than formation pressure an aqueousacidizing composition, maintaining said composition in contact with theformation strata for a time sufficient for the acid to chemically reactwith the acid-soluble components of the formation to etch or enlargepassageways therethrough thereby increasing substantially the flowcapacity of the said subterranean formation.

The novel acidizing composition of this invention comprises an aqueoussolution of a non-oxidizing mineral acid, such as hydrochloric,sulfuric, etc., having dissolved therein a vinylpyrrolidone polymer. Theamount-of acid present in the subject composition is such that it iscapable of reacting with acid soluble components of the fluid-bearingstrata.

A number of advantages result in employing the novel composition of thisinvention in acidizing calcareous fluid-bearing formations, namely:

1. The reaction rate of the acid with the formation acid-solubles, suchas carbonates, dolomites, etc., is greatly lessened. One of the mostserious problems encountered in the use of mineral acids as acidizingagents is the very rapid rate with which the known acidizingcompositions react with such acid-solubles so that little actual effecttakes place at any great distance from the well.

2. The post-precipitation of dissolved carbonates is prevented. Becauseof the nature of the dissolution reaction:

when pressure is released so that spent reaction products from theacidization process can flow back out or be pumped out, carbon dioxidegas can break out of solution, causing post-precipitation of calciumcarbonate. Such post-precipitation occurring within the formation matrixnear the bore hole can decrease permeability by plugging capillaries andresult in a lower production rate.

3. The viscosity of the acidizing fluid is increased. Such a viscosityincrease is characteristic of fluid-loss agents often used in acidizingformulations to cause more uniform formation penetration and minimizeleakage into zones of higher permeability or into natural fractures.

4. The cited polymer is highly effective in preventing swelling ofwater-sensitive clays or shales and thus formation damage of this typeduring acidization is largely avoided.

DETAILED DESCRlPTION OF THE lNVENTlON In its broadest embodiment themethod of the present invention comprises introducing into a subsurfacecalcareous formation an acid solution of a watersoluble vinylpyrrolidonepolymer wherein the said solution is maintained in contact with theformation for a time sufficient to chemically react with the formationso as to increase substantially the flow capabilities of the formationand to release carbon dioxideconcorni;

tantly whereby a beneficial effect due to the mutual miscibility ofcarbon dioxide in the fluid phases is realized as a reduction inviscosity and retentive capillary forces, while another beneficialeffect is realized in the form of increased formation energy, due to thepressure generated by the released carbon dioxide.

The average molecular weight of the vinylpyrrolidone polymers utilizedin the method of this invention generally will be from about l0,000 toabout 1,000,000 or more and preferably will be from about 100,000 toabout 400,000.

Highly advantageous results are realized with the method of thisinvention when water-soluble vinylpyrrolidone polymers having recurringunits of the formula:

wherein R, R,,, R and R are independently selected from the groupconsisting of hydrogen and alkyl radicals having from 1 to 5 inclusivecarbon atoms are employed. Useful alkyl radicals of 1 to 5 inclusivecarbon atoms include methyl, ethyl, propyl, butyl, pentyl, and isomericforms thereof. The sole limitation on the cited structure is that it besufficiently soluble in the aqueous acid. When R, R,,, R, and R ofFormula I above are each hydrogen, the resulting compound ispolyvinylpyrrolidone, i.e., poly-N-vinyl-2-pyrrolidone which is anespecially useful polymer.

Preferably, the acidic polymer solution of this invention is onecomprising an aqueous solution of about 3 to about 30 percent by weightof a non-oxidizing mineral acid, which may or may not include brine, andwhich contains dissolved therein between about 0.1 to about 10 percentby weight based 'on the total solution weight of the water-solublevinylpyrrolidone polymer.

Generally, the acidic polymer solution will contain an inhibitor toprevent or greatly reduce the corrosive attack of the acid on metal. Anyof a wide variety of compounds known in the art and employed for thispurpose can be used, e.g., certain compounds of arsenic, nitrogen orsulfur as described by Grebe et al. in U.S. Pat. No. 1,877,504 or arosin amine type as described in U.S. Pat. No. 2,758,970.,The amount ofthe inhibitor utilized is not highly critical and it may be variedwidely. Usually this amount is defined as a small but effective amount,e.g., from 0.02 percent to about 2.0 percent by weight or more of theacidic polymer solution.

The preparation of vinylpyrrolidone polymers suitable for use in theacidizing composition of this invention is described in Kirk-Othmer,Encyclopedia of Chemical Technology, Vol. 1, Second Edition,lnterscience Publishers, N.Y., 1963, p. 205. Such vinylpyrrolidonemonomers can be synthesized by reacting a carboxylic acid amide of theformula:

such as 2-pyrrolidone, 3-methyl-2-pyrrolidorfe,4,4-diethyl-2-pyrrolidone, 5-isobutyl-2-pyrrolidone,4-methyl-2-pyrrolidone, 3-ethyl-2-pyrrolidone, 3-methyl-S-ethyl-2-pyrrolidone, etc., with acetylene or an acetylenicderivative of the formula:

wherein R, and R have the same meaning as previously described, underpressure at a temperature of about 130 to about 160C. and in thepresence of the alkali metal salts of these acetylenic compounds ascatalysts. Polymerization of the resulting monomers can be conducted bymethods well known in the art.

In carrying out the method of this invention a solution of from about 3to about 30 percent by weight of the non-oxidizing mineral aciddissolved in water is first prepared. An inhibitor to prevent corrosionof acid on the metal equipment associated with the well is usually addedwith mixing in the next step. The vinylpyrrolidone polymer in an amountwithin the stated concentration range is then admixed with the aqueousacid solution employing a blender. The polymer dissolves rather rapidlyin the acid solution and the thusprepared composition is forced, usuallyvia a suitable pumping system, down the well bore and into contact withthe formation to be treated. As those skilled in the art will readilyunderstand, the pressure employed is determined by the nature of theformation, viscosity of the fluid, and other operating variables. Theacidization method of this invention may be carried out at a pressuresufficient merely to penetrate the formation or it may be of sufficientmagnitude to overcome the weight of the overburden and create fracturesin the formation. Propping agents, to prop open the fractures ascreated, for example 20 to 60 mesh sand, in accordance with knownfracturing procedures, may be employed in admixture with the aqueousacidic solution. Generally, it is advisable to allow the aqueous acidicpolymer solution to remain in contact with the formation until the acidtherein has been substantially depleted by reaction with theacid-soluble components of the formation. After this, the substantiallyspent treating solution is reversed out of the well, i.e., it is allowedto flow back out or to be pumped out of the formation. Further, as thoseskilled in the art will understand, the concentration of the polymer andacid should be chosen to provide an acidizing fluid of the desiredrheological properties. Similarly, the appropriate molecular weightpolymer is selected on this same basis.

The following is a description by way of examples of three series oftests whereby the effectiveness of the present invention has beendemonstrated.

In this first series of tests the reaction rate of a cylindrical Austinchalk core (2.2 cm in length, 2.2 cm. in diameter) with solutions of 1percent and 0.5 percent by weight of polyvinyl-pyrrolidone of molecularweight 360,000, where R,R,,,R,, and R are hydrogen atoms and intrinsicviscosity of 1.61 dissolved in a percent by weight aqueous hydrochloricacidsolution was determined. The results are summarized in the Table 1which follows:

TABLE 1 ppm dissolved CaCO,

Time 15% l-ICI 0.5% A* in 15%I-IC1 1% A in 15% HCI (min.)

Additive A polyvinylpyrrolidone having an average molecular of 360,000,R and R are hydrogen atoms; and instrinsic viscosity of 1.61.

From the data presented in the above table, it is evident that thepolyvinylpyrrolidone, when dissolved in a mineral acid widely employedin acidizing procedures, is effective in greatly reducing the rate ofattack by said acid on a limestone or chalk formation.

In a second series of tests the viscosity of solutions of 0.5 percentand 1 percent by weight of the same polyvinylpyrrolidone polymeremployed in the first series of tests (i.e.,Additive A) dissolved in 15percent by weight aqueous hydrochloric acid over extended periods oftime was measured. Many polymeric materials hydrolyze or otherwisedecompose in mineral acids over relatively short periods of time withaccompanying simultaneous reduction in viscosity and thereforeeffectiveness in well acidization operations.

Accordingly, the viscosity of the above-described polymer solutions inhydrochloric acid was measured over a period of 28 days. The results areset forth in Table 2.

The results depicted in Table 2 indicate that little, if any,decomposition of the polymer occurs over a period of 28 days.

In this third series of tests an Ostwald viscometer was used to measurethe viscosity (relative to 15% HCI) of various solution of the samevinylpyrrolidone polymer employed in the first series of tests (i.e.,Additive A) in 15 percent by weight I-ICI at room temperature. Thefollowing table summarizes these results.

TABLE 3 RELATIVE VISCOSITY Concentration of additive in 15% HCI) 0COMPOSITION 15% HCI Solution of Additive A in I-ICI Solution of AdditiveB inHCI Additive B-polyvinylpyrrolidone having an average molecularweight of 40,000, R and R are hydrogen atoms and intrinsic viscosity of0.225.

These data demonstrate that dissolution of the cited polymer in 15%I-lCl produces a remarkable increase in viscosity.

EXAMPLE 1 A well drilled in Gregg County, Texas in a tight limestoneformation was treated with an aqueous acidic polymer composition of thisinvention in order to stimulate oil production. in preparing to treatthe producing formation of the well a packer was set at 7253 feet aboveperforations located in the interval 7285-95 feet. A solution of 1percent by weight polyvinylpyrrolidone having a molecular weight of360,000 and an intrinsic viscosity of 1.61 was prepared by dissolvingcompletelyv 500 pounds of the polymer in 6000 gallons of 15 percent byweight hydrochloric acid using cyclic turbulent circulation. Aconventional corrosion inhibitor and non-emulsifying agent were presentin the acid.

In the first part of the stimulation operation, a pad of 2,100 gallonsof lease water containing 20 gallons of a scale inhibitor initially toprevent post-precipitation of carbonates dissolved in the subsequentacidizing process was pumped into the formation. In the next step, 1000gallons of conventional 15 percent HCl was pumped into the formation toremove scale in the vicinity of the well bore. in the third step, 4,000gallons of the acidizing mixture previously described was pumped intothe formation Finally, the aqueous acidic polymer solution was displacedinto the formation by pumping an additional 16,000 gallons of leasewater into it. The well was shut in nine days after treatment and at theend of that time the production was measured and found to be 5 bbl/dayof oil with 140 bbl/day of water as compared to a production rate of 3bbl/day of oil and bbl/day of water prior to the acidization treatment.After 33 days had elapsed production was 14 bbl/day of oil and 130bbl/day of water.

EXAMPLE H In this example, a well drilled in Caddo Parish, Louisianainto a tight limestone formation was treated with a quantity of the sameacidic aqueous polymer solution as utilized in Example I. In this well,open end tubing was set at 2220 ft. Perforations were in the intervals2234-2240 ft. and 2262-2266 ft. which contains 72 shots. The procedurefollowed in stimulating production of the producing formation andthis'well was thesame as that described in Example I except that noconventional 15% HCl was used initially, 1,000 gallons of the acidicaqueous polymer solution was employed in treating the formation and only1500 gallons of lease water flush was achieved becaue of mechanicalproblems at the well head. A total of 75 balls were used as diverters inthe retarded acid stage of this treatment operation, the purpose of theballs being to divert the acid to assure that it enters allperforations.

After treatment, the well was shut in for 2 days and at that timeproduction was measured and found to be 34 bbl/day of oil and 107bbl/day of water whereas, prior to the stimulation treatment of thisexample, the production was bbl/day of oil and 32 bbl/day of water.After 114 days had elapsed, production was 18 bbl/day of oil and 89bbl/day of water.

What is claifriedis: W

l. A method of increasing the production of fluids from a subterraneanfluid bearing formation having present therein acid-soluble componentsand being without water-sensitive shales or clays comprising injectingdown the well bore penetrating said formation and injecting therefrominto said formation under a pressure greater than the formation pressurean aqueous well acidizing composition comprising an aqueous solution ofa mineral acid having dissolved therein a vinylpyrrolidone polymer, thesaid acid being present in an amount capable of reacting with theacid-soluble components of a subterranean fluid-bearing formation andwherein the said polymer comprises recurring units of the formula:

wherein R, R R and R are independently selected from the groupconsisting of hydrogen and alkyl of from 1 to 5 inclusive carbon atomsmaintaining said composition in contact with the formation for a timesufficient for the acid to chemically react with the acid solublecomponents of the formation to etch passageways therethrough therebyincreasing substantially the flow capacity of the said subterraneanformation.

2. The method of claim 1, wherein the said formation is ahydrocarbon-bearing formation.

3. The method of claim 1, wherein the said composition is injected downthe well bore penetrating said formation under a pressure greater thanthe formation pressure but less than the pressure required to createfractures in the formation.

4. A method of increasing the production of fluids from a subterraneanfluid bearing formation having present therein acid-soluble componentsand being without water-sensitive shales or clays comprising injectingdown the well bore penetrating said formation and injecting therefrominto said formation undera pressure greater than the formation pressurean aqueous well acidizing composition comprising an aqueous solution ofa mineral acid having dissolved therein a vinylpyrrolidone polymerpresent in a concentration of from about 0.1 to about 10 percent byweight, the said acid being present in an amount capable of reactingwith the acid-soluble components of a subterranean fluid-bearingformation and wherein the said polymer comprises recurring units of theformula:

wherein R, RfR and R are independently selected from the groupconsisting of hydrogen and alkyl of from 1 to 5 inclusive carbon atoms,maintaining said composition in contact with the formation for a timesufficient for the acid to chemically react with the acidsolublecomponents of the formation to etch passageways therethrough therebyincreasing substantially the flow capacity of the said subterraneanformation.

5. A method of increasing the production of fluids from a subterraneancalcareous fluid-bearing formation having present therein acid-solublecomponents and being without water-sensitive shales or clays comprisinginjecting down the well bore penetrating said formation and injectingtherefrom into said formation under a pressure greater than theformation pressure an aqueous well acidizing composition comprisingabout 3 to about 30 percent by weight of hydrochloric acid havingdissolved therein from about 0.1 to about 10 percent by weight based onthe total weight of the composition of polyvinylpyrrolidone having anaverage mo 10 being without water-sensitive shales or clays comprisinginjecting down the well bore penetrating said formation and injectingtherefrom into said formation under a pressure greater than theformation pressure an aqueous well acidizing composition comprisingabout 3 to about 30 percent by weight of hydrochloric acid havingdissolved therein from about 0.1 to about 10 percent by weight based onthe total weight of the composition of polyvinylpyrrolidone having anaverage molecular weight of about 100,000 to about'400,000, maintainingsaid composition in contact with the formation for a time sufficient forthe acid to chemically react with the acid-soluble components of theformation to etch passageways therethrough thereby increasingsubstantially the flow capacity of the said subterranean formation.

2. The method of claim 1, wherein the said formation is ahydrocarbon-bearing formation.
 3. The method of claim 1, wherein thesaid composition is injected down the well bore penetrating saidformation under a pressure greater than the formation pressure but lessthan the pressure required to create fractures in the formation.
 4. Amethod of increasing the production of fluids from a subterranean fluidbearing formation having present therein acid-soluble components andbeing without water-sensitive shales or clays comprising injecting downthe well bore penetrating said formation and injecting therefrom intosaid formation under a pressure greater than the formation pressure anaqueous well acidizing composition comprising an aqueous solution of amineral acid having dissolved therein a vinylpyrrolidone polymer presentin a concentration of from about 0.1 to about 10 percent by weight, thesaid acid being present in an amount capable of reacting with theacid-soluble components of a subterranean fluid-bearing formation andwherein the said polymer comprises recurring units of the formula:
 5. Amethod of increasing the production of fluids from a subterraneancalcareous fluid-bearing formation having present therein acid-solublecomponents and being without water-sensitive shales or clays comprisinginjecting down the well bore penetrating said formation and injectingtherefrom into said formation under a pressure greater than theformation pressure an aqueous well acidizing composition comprisingabout 3 to about 30 percent by weight of hydrochloric acid havingdissolved therein from about 0.1 to about 10 percent by weight based onthe total weight of the composition of polyvinylpyrrolidone having anaverage molecular weight of about 100,000 to about 400,000, maintainingsaid composition in contact with the formation for a time sufficient forthe acid to chemically react with the acid-soluble components of theformation to etch passageways therethrough thereby increasingsubstantially the flow capacity of the said subterranean formation.
 6. Amethod of increasing the production of fluids from a subterraneanlimestone fluid-bearing formation having present therein acid-solublecomponents and being without water-sensitive shales or clays comprisinginjecting down the well bore penetrating said formation and injectingtherefrom into said formation under a pressure greater than theformation pressure an aqueous well acidizing composition comprisingabout 3 to about 30 percent by weight of hydrochloric acid havingdissolved therein from about 0.1 to about 10 percent by weight based onthe total weight of the composition of polyvinylpyrrolidone having anaverage molecular weight of about 100,000 to about 400,000, maintainingsaid composition in contact with the formation for a time sufficient forthe acid to chemically react with the acid-soluble components of theformation to etch passageways therethrough thereby increasingsubstantially the flow capacity of the said subterranean formation.